Effects of catecholamines on isolated human colonic smooth muscle

Opposite effects of dopamine on the mechanical activity of circular and longitudinal muscle of human colon

BACKGROUND

Because dopamine (DA) has gained increasing evidence as modulator of gut motility, we aimed to characterize dopaminergic response in human colon, evaluating function and distribution of dopamine receptors in circular vs longitudinal muscle strips.

METHODS

Mechanical responses to DA and dopaminergic agonists on slow phasic contractions and on basal tone were examined in vitro as changes in isometric tension. RT-PCR was used to reveal the distribution of dopaminergic receptors.

KEY RESULTS

In spontaneous active circular muscle, DA induced an increase in the amplitude of slow phasic contractions and of the basal tone, via activation of D1-like receptors. DA contractile responses were insensitive to neural blockers or to atropine and inhibited by phospholipase C (PLC) pathway inhibitors. In precontracted circular muscle strips, DA, at the higher concentrations tested, caused a relaxant response via activation of D2-like receptors. In the longitudinal muscle, DA caused only muscular relaxation due to activation of D2-like receptors. DA relaxant responses were insensitive to neural blockers or to nitric oxide synthase inhibitor and reduced by a wide-spectrum K⁺ channel blockers. Transcripts encoding for all the dopaminergic receptor subtypes was observed in both circular and longitudinal preparations.

CONCLUSIONS AND INFERENCES

Dopamine is able to modulate contractile activity of the human colon. In the circular muscle layer, DA induces mainly muscular contraction activating non-neural D1-like receptors, coupled to PLC/IP3 pathway. In the longitudinal muscle layer, DA induces muscular relaxation acting on non-neural D2-like receptors leading to the increase in K⁺ conductance.

Laxation of critically ill patients with lactulose or polyethylene glycol: a two-center randomized, double-blind, placebo-controlled trial

OBJECTIVE

To study whether lactulose or polyethylene glycol is effective to promote defecation in critically ill patients, whether either of the two is superior, and whether the use of enteral laxatives is related to clinical outcome.

DESIGN

Double-blind, placebo-controlled, randomized study.

SETTING

Two tertiary intensive care units.

PATIENTS

Three hundred and eight consecutive patients with multiple organ failure were included when receiving mechanical ventilation and intravenous circulatory support and when defecation did not occur on day 3 after admission.

INTERVENTIONS

Thrice daily administration of lactulose, polyethylene glycol, or placebo until defecation occurred, to a maximum of 4 days.

MEASUREMENTS AND MAIN RESULTS

The number of patients with defecation during the study period was 32 of 103 (31%) for placebo, 76 of 110 (69%) for lactulose, and 70 of 95 (74%) for polyethylene glycol (p = .001 for lactulose and polyethylene glycol vs. placebo). Lactulose and polyethylene glycol-treated patients produced stools after a median of 36 and 44 hrs, respectively, compared with 75 hrs for the placebo group (p = .001 for lactulose and polyethylene glycol vs. placebo). Length of stay in the intensive care unit was a median of 156 hrs for the lactulose group, 190 hrs for the polyethylene glycol group, and 196 hrs for the placebo group (p = .001). Intestinal pseudoobstruction or Ogilvie's syndrome occurred in 4.1% of patients in the placebo group, 5.5% of patients in the lactulose group, and 1.0% of patients in the polyethylene glycol group. There was no difference in hospital mortality. Administration of morphine was associated with a longer time before first defecation, except in the polyethylene glycol group. For all groups, defecation within 6 days after admission was associated with a shorter length of stay.

CONCLUSIONS

Both lactulose and polyethylene glycol are more effective in promoting defecation than placebo. Patients receiving polyethylene glycol had a slightly lower incidence of acute intestinal pseudoobstruction, whereas length of stay was shorter in lactulose-treated patients. Morphine administration was associated with delayed defecation except in the polyethylene glycol-treated group. Irrespective of study medication, early defecation was associated with a shorter length of stay.

Preservative solution for freeze-storage of surgically excised human colon to enable study of smooth muscle function in vitro

We have compared the reactivity to carbachol and high potassium of circular smooth muscle isolated from segments of human colon which was freeze-stored in different preservative solutions for more than one month following surgical resection. Concentration-dependent contractions in response to carbachol were reduced in terms of both their sensitivity (pEC50) and reactivity (Emax), depending on the preservative solutions used. Similar reduction of reactivity to 100 mM KCl was also observed. The best responsiveness was shown when the tissue was freeze-stored in SFM101. It is concluded that the freeze-storage of surgically excised human colon in SFM101 or phosphate buffer solution for more than one month provided the best preservation of smooth muscle function for in vitro pharmacological examination.

Optimisation of isolation of richly pure and homogeneous primary human colonic smooth muscle cells

BACKGROUND

Inherent properties of gastrointestinal smooth muscle can be assessed using isolated cell suspensions. Currently available isolation techniques, based on short 2-h enzymatic digestion, however, present the disadvantage of low cellular yield with brief viability. These features are an important limiting factor especially in studies in humans in which tissue may not be available daily and mixing of samples is not recommended.

AIMS

To optimise the isolation procedure of cells from human colon to obtain a richly pure primary smooth muscle cell preparation.

METHODS

Slices of circular muscle layer, obtained from surgical specimens of human colon, were incubated overnight in Dulbecco's modified eagle's medium supplemented with antibiotics, foetal bovine serum, an ATP-regenerating system and collagenase. On the following day, digested muscle strips were suspended in HEPES buffer, and spontaneously dissociated smooth muscle cells were harvested and used either immediately or maintained in suspension for up to 72 h. Cell yield, purity, viability, contractile responses, associated intracellular calcium signals and RNA and protein extraction were evaluated and compared to cell suspensions obtained with the current short digestion protocol.

RESULTS AND CONCLUSION

The overnight isolation protocol offers the advantage of obtaining a pure, homogeneous, long-life viable cell suspension that maintains a fully differentiated smooth muscle phenotype unchanged for at least 72 h and that allows multiple functional/biochemical studies and efficient RNA extraction from a single human specimen.

An improved in vitro bioassay for the study of 5-HT(4) receptors in the human isolated large intestinal circular muscle

Recently, it was demonstrated that 5-HT induces relaxation of human colon circular muscle through activation of 5-HT(4) receptors and 5-HT(7) receptors. The aim of the current study was to develop a new in vitro bioassay of human colon that would facilitate the pharmacological analysis of 5-HT responses mediated solely by 5-HT(4) receptors. Contracting circular muscle strips with KCl (80 mM) yielded a stable contractile tension and, in contrast to muscarinic cholinoceptor agonists and histamine, a profound reduction of spontaneous contractility. This allowed the establishment of reproducible, fully-defined, agonist concentration-response curves by cumulative dosing. Under these conditions, 5-HT induced a concentration-dependent relaxation (pEC(50) 7.31, Hill slope 0.91). Neither methysergide (10 microM) nor granisetron (1 microM) affected the 5-HT-induced relaxation, suggesting that 5-HT(1), 5-HT(2), 5-HT(3), 5-ht(5), 5-HT(6) or 5-HT(7) receptors are not involved. The lack of effect of tetrodotoxin (0.3 microM) indicated a direct effect of 5-HT on the smooth muscle. The selective 5-HT(4) receptor antagonists GR 113808, GR 125487 and RS 39604 competitively antagonized the 5-HT-induced relaxation (pK(B) 9.43, 10.12 and 8.53, respectively). SB 204070 (1 nM) produced a rightward shift (pA(2) 10.34) and depression of the 5-HT curve. These affinity estimates are similar to those previously reported for 5-HT(4) receptors. The selective 5-HT(4) receptor agonists, prucalopride and R076186, induced relaxations (pEC(50) 7.50 and 7.57, respectively), that were blocked by GR 113808 (3 nM), yielding pA(2) estimates of 9.31 and 9.21, respectively. To summarise, in KCl (80 mM)-contracted muscle strips, 5-HT induces relaxation through activation of a homogeneous smooth muscle 5-HT(4) receptor population. This new bioassay allows the focused, pharmacological characterization of human colonic 5-HT(4) receptors in vitro.

In vitro inhibition of human colonic motility with SR 59119A and SR 59104A: evidence of a beta3-adrenoceptor-mediated effect

The new beta3-adrenoceptor is present in the gastrointestinal tract of various species. This study aimed to show that this receptor modulates human colonic motility in vitro. We used circular muscle strips from the human colon suspended in single organ baths containing Krebs solution and subjected to an initial 1.5-2 g tension. We measured the effects of different beta3-adrenoceptor agonists, including SR 59104A (N-[(6-hydroxy-1,2,3,4-tetrahydronaphthalen-(2R)-2-yl)methyl]-(2 R)-2-hydroxy-2-(3-chlorophenyl)ethanamine hydrochloride), SR 59119A (N-[(7-methoxy-1,2,3,4-tetrahydronaphthalen-(2R)-2-yl)methyl]-(2R) -2-hydroxy-2-(3-chlorophenyl)ethanamine hydrochloride), BRL 37344 (R,R + S,S) [4-[2-[[2-(3-chlorophenyl)-2-hydroxyethyl]-amino] propyl] phenoxy] acetic acid), and of isoprenaline and salbutamol in the absence or in the presence of propranolol alone or in combination with the beta3-adrenoceptor antagonist SR 59230A (3-(2-ethylphenoxy)-1-[(1S)-1,2,3,4-tetrahydro-naphthalen-1- ylamino]-(2S)-2-propanol oxalate) on amplitude of spontaneous contractions. To evaluate a possible beta2-adrenoceptor-mediated effect, we studied the action of these compounds on human isolated bronchi. On the human isolated colon, SR 59119A, SR 59104A and isoprenaline reduced the initial amplitude of spontaneous contractions by 60%. The curves obtained in the presence of antagonists suggested an action mediated by beta3-adrenoceptor stimulation, since propranolol did not antagonize the action of SR 59119A and SR 59104A, whereas the combination of propranolol and SR 59230A significantly displaced the concentration-response curve of these agonists to the right. This study provides pharmacological evidence of modulation of human colonic motility, and especially of the amplitude of spontaneous contractions, by the atypical beta-adrenoceptor, the beta3-adrenoceptor.